Remote laser vibration sensing (also known as vibrometry) is finding application in a multitude of scenarios such as target recognition in civilian and combat scenarios, mine detection, etc., whereby a target can be detected based upon an effect a motion of the target has upon an incident pulsed beam. In short range applications (e.g., a few meters distance), laser vibrometry can be utilized to test mechanical structures with respect to their vibration characteristics. At longer ranges (e.g., in the order of several kilometers), effects such as whether a vehicle is positioned with its engine idling can be determined. In operation, a laser beam associated with a known frequency is directed towards an object that is subject to vibration (e.g., the object is vibrating). The laser beam is reflected from the object, whereby over time, the vibration of the object can cause the reflected signal to scatter. Thus, as the object vibrates, a Doppler effect can arise in the scattered signals that are reflected by the object. By determining a shift in the Doppler frequency, and accordingly, the vibration frequency, it is possible to determine a magnitude of vibration, e.g., due to the motion of the idling engine.
To facilitate object detection (e.g., by the Doppler-shifted frequency of a scattered or returned signal) over longer ranges can require a laser beam to have a high magnitude of power in a very narrow frequency band compared with a laser beam that is utilized for shorter ranged applications. A laser beam reflected from an object(s) having an uneven surface(s) can result in dispersion of the reflected energy over wide spatial angles and, thus, the detected power can be significantly reduced, especially for a long-range detection. Such reduction in power can necessitate utilization of a high power laser source. Vibrometry at the longer ranges can often only be achieved using lasers with a very narrow band (e.g., in the 1 kHz band range), in both continuous wave (CW) mode and pulsed mode. However, utilizing higher power lasers can also introduce an issue of non-linear effects, where such non-linear effects can act to spectrally broaden the laser beam. Accordingly, such non-linear effects can further render it difficult to demodulate a vibration signature in a reflected pulse(s) as the broadening ‘washes out’ the vibration signature. Hence, while technology exists to perform laser vibrometry over an extended distance, the ability to accurately determine the vibration frequency of an object can be hindered by broadening of a laser signal with regard to loss in focused power and also washing out of the vibration signature.